Steam-engine and like indicator.



B. HOPKINSON. STEAM ENGINE AND LIKE INDICATOR.

APPLICATION FILED FEB. 14, 1907.

6 SHEETS-SHEET 1 Patented Feb. 16, 1909.

in-m ATTU RNEY 5 B. HOPKINSON. STEAM ENGINE AND LIKE INDICATOR.

APPLICATION FILED PEB.14, 1907. 912,680, Patented Feb. 16, 1909.

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.B. HOPKINSON.

STEAM ENGINE AND LIKE INDICATOR. APPLIOATION FILED FEB. 14, 1907'.

91 2,680, Patented Feb. 16, 1909.

ATFURNEYS B. HOPK'INSON.

STEAM ENGINE AND LIKE INDICATOR. APPLICATION r1131) 213.14, 1907.

912,680. Patented'Feb. 16, 1909.

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B. HOPKINSON. STEAM ENGINE AND LIKE INDICATOR. APPLICATION FILED FEB.14, 1907. v 91 2,680, Patented Feb. 16, 1909;

5 SHEETS-SHEET 6.

BERTRAM HOPKINSON, OF CAMBRIDGE, ENGLAND.

STEAi I-ENGINE AND LEKE INDICATOR.

Specification or Letters Patent.

Patented Feb. 16, 1909.

Application filed February 14-, 1907. Serial No. 357,508.

To all whom it may concern.

Be it known that i, BERTRAM Hornnvson, a subject of the King of GreatBritain, residing at the Engineering Laboratory, Qambridge, in thecounty of Cambridge, England, engineer, have invented certain new anduseful Improvements in Steam-Engine and Like Indicators, of which thefollowing is a specification.

This invention relates to indicators for recording the pressure inreciprocating engines. Hitherto indicators for this purpose have been oftwo types. First, there is a well known steam engine indicator in Wnicha piston in a chamber communicating with the engine cylinder is causedto compress a spring. This movement of the piston is magnified by asystem of levers and is transferred to a pencil which is pressed againsta paper carried on a reciprocating drum. T he drum is connected to thepiston of the engine and is thus caused to move at right angles to themotion of the pencil, to an extent proportionate to the motion of theengine piston. This type of indicator is not suitable for use with highspeed engines on account of the inertia of. the moving parts. Moreover,f om the same cause it is unable to follow accurately the very rapidvariation of pressure occurring at the moment of explosion in a gasengine, even at slow speeds. in the second type of indicator in commonuse the piston is replaced by a diaphragm which forms part of a wall ofa chamber connected to the engine cylinder. The movement of thediaphragm produced by the varying pressure is transferred to a mirrorwhich it causes to turn about one axis, while other mechanism connectedto the piston or crank shaft of the engine causes the mirror to turnabout a second aXis perpendicular to the first through an angleproportionate to the motion of the engine piston. A beam of light from afixed source is reflected from the mirror and is brought to a focus on aground glass screen. The point or spot of light so produced traces out acurve whose ordinates are proportionate to the motion of the diaphragm,while the abscissa; are proportionate to the motion of the enginepiston. This second form of indicator is suitable for use with enginesof high speed, since the inertia of its moving parts is small; butit'suffers from the disadvantage that the movement of the diaphragm andthe corresponding movement of the mirror and the spot of light, are notproportionate to the change of pressure. The means hitherto used forproducing that movement of the mirror and the spot of light, whichcorresponds to the motion of the engine piston, are inaccurate,delicate, and apt to get out of order. Moreover, diagrams procuced inthis way cannot well be seen unless a very powerful source of light beused or the room be darkened, and it is difficult or impossible to makeaccurate measurements of the diagrams except by means of photography.

The chief object of my invention is to combine the advantages of bothtypes of indicator, and to produce an apparatus which is simple toconstruct and easy to apply to the engine; and in which, at the sametime, the moving parts are light and move through very small distances,so that the a )paratus can correctly respond to eXtremey rapid changesof pressure, and readily coo aerate with the means hereinafter describedor the easy observation and measurement of the diagrams obtained. Forthis purpose I make use of a mirror which reflects a beam of light andwhich is tilted about two axes in its plane at right angles to oneanother as in the diaphragm type of indicator alluded to above. Themovement about the first aXis is produced by the pressure acting upon aspring controlled piston and that about the second axis by the enginepiston acting upon an oscillatory frame which is capable of movementabout the axis of the spring controlled piston. The first mentioned movement may be called the pressure motion, and the second the pistonmotion.

In order to observe and measure the optical diagram obtained, I causethe beam of light emanating from a fixed luminous point or source oflight to be reflected from the mirror to a lens of considerable diameterand thence to reach the eye of the observer without falling u on asemi-opaque screen.

In order t at my said invention may be clearly understood and readilycarried into effect I will describe the same with reference to theaccompanying drawings, in which z- Figure l is a side elevation, Fi 2 aplan, and Fig. 3 a vertical section on the line 11 of Fig. 1, showing aconvenient form of my improved apparatus. Figs. 4 and 5 are elevationstaken at right angles to each other and show means that may be employedfor imparting the piston motion to the apparatus. Fig. 6 is a sideelevation and Fig. 7 a vertical section on the line 22 of Fig. 6 showinga modified form of the apparatus for enabling it to indicate pressurebelow as well as above that of the atmosphere. Fig. 8 is a verticalsection showing a further modification in which the apparatus isprovided with two mirrors. Fig. 9 shows diagrammatically one method ofeffecting observation and measurement of the optical diagrams obtained,Fig. 10 shows a modified method. Fig. 11 is a plan view and Fig. 12 anelevation showing diagrammatically the general arrangement of the meansfor optica lly observing the diagram obtained, and Fig. 13 is a frontelevation of the transparent screen on fvhlilch the diagram is traced bythe spot of Referring more particularly to Figs. 1 to 8, A is a metalblock which is firmly screwed into the wall B of the engine cylinder. Itis bored out along its axis, and in the bore (which communicates withthe interior of the engine cylinder) a piston C is fitted to slideeasily. The upper part of the block A is turned conical. Fittingaccurately over this conical part is a frame D which carries a springblade E, which is so placed that in its initial position it just'clearsthe upper end of the piston C. Carried on the frame D are two verticalpieces of spring G, G, and in holes at the upper ends of these a spindleH is pivoted. Firmly fixed to the spindle is the mirror I, its planebeing roughly parallel to the axis of the block A. At right angles tothe spindle and fixed to it is a short arm J (Figs. 2 and 3) and this isconnected at its outer end to the middle point of the spring blade E, bymeans of a short piece of straight and fairly stifi spring K. The blockD is held upon the conical seating of the block A by. means of a nut andlock-nut L, L, so that it can turn upon the cone bearing of the block Awithout shake. The piston movement is given to the mirror by causing theframe D with the attach ed mirror to oscillate about the axis of theblock A in time with the engine piston and through an angleproportionate to the travel of the said piston. This oscillatorymovement may be obtained by the means diagrammatically shown by Figs. 4and 5 in which an eccentric N is fixed to the crank shaft M of theengine, the throw of the eccentric being relatively to its diameter inthe ratio of the stroke of the engine to the length of its connectingrod. A lever O pivoted at P, carries at its lower end a roller Q, whichis pressed by a spring or other convenient means against the eccentric.As the crank shaft rotates, the motion of any point R on the lever willthen be a reduced copy of the motion of the engine piston. To transferthe motion to the indicator, a lever S clamped to the frame D is jointedto the lever O at the point R. The lever S and with it I i i the frame Dthen oscillate through an angflie proportionate to the motion of thepoint and therefore to the motion of the engine piston. in order to takeup the slight curvature of the path of the point R, the lever S mayconveniently consist of a flat piece of steel which will bend in avertical but not in a horizontal plane, the joint at B being a balland-socket joint to provide for the vertical motion.

The pressure movement, in the example shown, is obtained by the pressureof the piston C upon the spring E, in consequence of which the latter isdeflected vertically through a small distance proportionate to thepressure in the engine. The end of the arm J is pushed upward by thespring K through a distance equal to the deflection of the spring E, andthe spindle H accordingly turns through an angle proportionate to thepressure. A beam of light from a fixed source after reflection from themirror will therefore turn in a vertical plane through an angleproportionate to the pressure, and in consequence of the rocking motiongiven to the frame D and the mirror about a vertical axis, the said beamwill also turn in a hori- Zontal plane through an angle proportionate tothe motion of the engine piston. If the beam of light be focused, afterreflection, upon a ground glass screen, the resulting spot of light,will, in consequence of the two combined motions of the beam, travel ina visible path which is the indicator diagram of the engine, theabscissze being proportional to the travel of the engine piston and theordinates to the corresponding pressures. In order to facilitate theobservation of the diagrams thus obtained I resort to the methods suchas illustrated in Figs. 9 and 10, which are hereinafter described.

The means for holding the spring E in the frame D are such that it isheld at definite points, so that the spring is able to slide, in thedirection of its length and cannot therefore be put into excessivetension by unequal expansion of the frame D and the spring E. In themethod of holding the spring shown in Figs. 1 and 3 these objects areobtained by passing the spring E through slots in the plates F, F. Thespring fits easily in these slots, and screws F, F, press on its uppersurface so as to cause it to rest with slight pressure on the bottom ofthe slots. These screws also bend the spring slightly. The spring isthus held at four definite points, two above and two below, those abovenot being opposite to those below, and the pressure on these pointsbeing suficient to hold the spring in position, but leaving it free toexpand and contract longitudinally.

The method of producing the pressure motion may be considerably variedwithout departing from the essential principles of my invention. Forexample, the spring E may be held only at one end, instead of at bothends, or a spring of other form, such as a spiral spring, may besubstituted for it. Again, the means of communicating tlie motion of thespring to the mirror may be me dified In the form of apparatusillustrated in Figs. 1 to 3 the piston is only able to exert pressureupon the spring greater than atmospheric pressure and consequently theapparatus will not register pressures below the atmospheric. For manypurposes tlis is suflicient, but in some cases it is desired to indicatesuction pressures as well as compression pressures and to meet suchcases I arrange that the piston shall be able to exert a pull as well asa push upon the spring in the manner shown in Figs. 6 and 7. Tie pistonC is firmly attached by means of the screw T to the block or stirru U.Tl'zrough a square hole in the latter the spring E is passed. he screw Vis adjusted so as to leave a very small clearance between its end andthe spring E. The spring E is then free to move slightly within theblock U, so that the piston is enabled to take its proper axial positionwithout any lateral pressure wh'eh would cause it to bind in the bore inwhich it slides. In this case the attachment of the spring to the mirroris placed slightly to one side of the piston as shown in Fig. 6. A ballW may be placed underneath the spring in the block U in order tofacilitate the relative movement of the spring and the piston.

In cases where the vibration of the engine hen running is excessive andcauses angular motion of the mirror comparable with that caused by thepressure, I make use of two mirrors in the manner shown in Fig. 8. Oneof these is the ordinary mirror I of the indicator; the other I is fixedto the frame, its plane being approximately at right angles to that oftlxe mirror I. Light from a fixed source falls upon the mirror I asshown by the dotted line with arrows; it is then reflected onto themirror-I and thence again reflected. The beam as finally reflected willreceive the piston movement and the pressure movement just as in thecase of the type of indicator illustrated in Figs. 1 to 3, but inasmuchas rotation of the apparatus as a whole about an axis parallel to thatof he mirror I, will be accompanied by an equal rotation of the mirrorI, such rotation will produce no change in the direction of the beam asfinally reflected from I; just as in a sextant the reflection from thetwo mirrors in succession is unaltered by movement of the instrument asa whole. 1

Thus in tee indicator the movement observed in the reflected beam willbe due to the pressure only, any movement of the mirror caused byvibration or by shake of the apparatus on the block A being eliminatedby the use of the two mirrors.

It will be understood that, although for convenience of description theindicator has been described as with a vertical axis so that hemovements of the mirror are respectively horizontal and vertical, thisvertica. position of the axis is not essential. The indicator may beplaced in any convenient position and the piston movement of the mirrorwill be about the axis of the bore, and tie pressure movement about anaxis at right angles thereto.

Referring now to Figs. 9 to 13 particularly to 9, the beam of lightcoming from a fixed luninous point or source a, falls upon 1. i J p v rV I L r i the aim osaid plane mirror I. Ihe renected beam is caused tofall upon a com ex lens Z, placed at a distance from the mirror equal toits own focal length, and to strilre said lens at the point 0. T hislens must be of such size that in the extreme positions of the mindetermined by the varying motions of the engine piston and the pressurewithin the c i der, the beam always stril'es the lens. lens causes theparallel beam ol' light to i i verge to a point (X which, since the sourdistant, will be a little beyond the focal plane of the lens. Since themirror is a 1 the other principal locus of the lens the direction of thebeam 0, (Z alter passing the lens, is parallel to the axial line I fjoining the center of the lens to the mirror, and it a semhopaque screenwere placed in the plane d j'tlie indicater diagram of the engine wouldbe traced thereon. In order to make this dia' "an-r readily visible, Ido not employ a ,and glass or other semi-opaque screen in the plane tlf,but l place a second lens Z at a distance from the plane (Z f equal toits own focal length, which is short enough to give considerablemagnification. This second lens is of approximately the same diameterthe first so that the rays from the first lens meet it in all positionsof the mirror. Altar passing the second lens the beam of lie' i c d 0becomes a parallel be and this parallr beam meets the axis I f at thepoint 7 which i. a principal focus of the second lens. At this point theeye of the observer is placed and sees through the eye piece P themagnified image of the diagram formed in the plane (if. Sincepractically no light is lost in the pas-- from the mirror to the eye, avery w source of light is suflicient to make the i lagram visible.

To provide [or the measurement oi" the diagram, I mayplace in the plane(Zf a transp ent screen Z Figs. 11, 12 and havin ured upon its surfacehorizontal and TO '0 scales. These scales will be seen at the same timeas the diagram and magnified to the same extent, and the diagram canaccordingly be very easily compared with them.

The source of light, lenses and screen may conveniently be carriedadjustably on the same frame X which may be mounted on a stand or pillarX which is adjusted in position with respect to the engine cylinderandthe mirror indicator fitted thereto, until the correct position forobtaining and viewing the diagram is found.

Although in the above description and in Fig. 9, I have shown a planemirror with a point source of light and have shown the first lens placedat a distance from the mirror equal to its own focal length, and thesecond lens at a distance from the focal plane of the first equal to itsfocal length, these details are not essential to my invention, which, inits broader scope consists in catching the rays reflected from theindicator mirror up on a lens placed at some considerable distance fromthe mirror, and in collecting the rays emerging from the lens by meansof a second lens which directs them to the eye of the observer.

Referring now to Fig. 10 in this case the rays from the source of lighta fall upon a convex lens Z before reaching the mirror I. A second lensZ is placed at such a distance from the mirror and is of such focallength that the point g, which is the focus of "his lens conjugate tothe mirror I, will be distant about fifteen inches from the lens. Thislens Z is, as in the previous case, of such diameter that the beam oflight reflected from the mirror always falls upon it. The lens Z is ofsuch focal length and is so placed in relation to the mirror B and theluminous point or source A, that the rays from that point or source cometo a focus near to the lens Z after reflection from the mirror, that isto say, the distance al and the sum of the distances Z l and I Z areconjugate focal distances for the lens Z. Under the arrangementdescribed, if a ground glass screen were placed in the position of thelens Z it would receive the indicator diagram of the engine in the wellknown manner; the effect however of substituting the lens Z for thescreen is to refract the beam falling upon the latter, in any positionofthe mirror, to the point g on the axis of the lens, at which point theeye of the ob server is placed. Since the distance of this point fromthe lens, (viz :about fifteen inches) is a convenient distance forfocusing the eye, and since the rays composing the beam in any positionproceed from a point on the lens as shown in the figure, the d gram willbe visible as a bright line of light. A screen bearing scales as in theprevious case may be placed close to the lens Z with which scales thediagram may be compared as before stated. In place of the convex lens Zand a plane mirror I a concave mirror might be used of such focal lengthas to bring the beam from the source a to focus upon the lens P.

It is obvious that these methods of observing diagrams may be applied toany form of indicator in which a beam of light from a mirror is causedto move in one direction in accordance with the pressure and in aperpendicular direction in accordance with the motion of the enginepiston. The said methods may also be used in connection with mirrorindicators employed with other apparatus than engines, that is to say,any apparatus that will operate to impart continuous cyclical motion tothe mirror about two axes at right angles.

in using the apparatus in connection with internal combustion engines anelectric spark gap a (Fig. 11) may be arranged in series with theigniting spark in the engine and close to the source of light. An imageof the spark at the time when it passes is then formed near that of thesource of light and this image is easily seen by the observer; thetiming of the spark can thus be investigated very conveniently.

What I claim and desire to secure by Letters Patent of the United Statesis 1. in apparatus for optically producing indicator diagrams, thecombination of a conical block having a bore communicating with thesource of pressure, a spring-controlled piston located in said bore, aframe mounted on said conical block, a mirror pivotally mounted on saidframe, means also carried by said frame whereby said piston impartsangular displacement to the mirror in one plane, and means whereby saidmirror is rocked about the axis of said piston in a plane perpendicularto the first mentioned plane, for the purposes specified.

2. in apparatus for optically producing indicator diagrams, thecombination of a piston subject to varying pressure, a frame locatedaxially with respect to said piston, a mirror pivotally mounted on saidframe, a bar spring mounted transversely in said frame so as to bearloosely on said piston, and to be deflected thereby at its center, meansfor oscillating said frame about the axis of said piston in one plane,and means whereby said spring imparts angular displacement to the mirrorin a plane perpendicular to the first mentioned plane for the purposespecified.

3. In apparatus for optically producing indicator diagrams, thecombination of a piston subject to varying pressure, a frame locatedaxially with respect to said piston, a mirror pivotally mounted on saidframe, a bar spring ada ted to bear loosely on said piston and to codeflected thereby, means whereby said spring is mounted transversely inthe frame at four points so as to be free at the ends and capable ofslight axial movement, means for oscillating said frame about the axisof said piston in one plane, and means whereby said spring impartsangular displacement to the mirror in a plane perpendicular to the firstmentioned plane for the purpose specified.

4. in apparatus for optically producing indicator diagrams, thecombination of a conical block having a bore communicating with thesource of pressure, a pis on located in said bore, a frame mounted onsaid block, a mirror pivotally mounted on said frame, a spring mountedtransversely in said frame so as to bear loosely on said piston and bedeflected thereby, means for oscillating said frame about the axis ofsaid piston in one plane, and means whereby said spring imparts angulardisplacement to the mirror in a plane perpendicular to the firstmentioned plane, for the purposes specified.

5. In apparatus for optically producing indicator diagrams, thecombination of a piston subject to varying pressure, a frame mountedaxially with respect to said piston, a mirror pivotally mounted on saidframe, a bar spring mounted transversely on said frame so as to bearloosely on said piston and to be deflected thereby at its center; meansalso carried by said frame whereby said spr ng imparts angulardisplacement to the mirror in one plane, means for oscillating saidframe about the axis of said piston in a plane perpendicular to thefirst mentioned plane, a source of light directed onto said mirror, andmeans whereby the reflected rays that trace out the diagram are directedduring all positions of the mirror to the observers eye, for the purposespecified.

6. in apparatus for optically producing indicator diagrams, thecmnbination of a spring-controlled piston subject to varying pressure, aframe mounted axially with respect to said piston, a mirror pivotallymounted on said frame, means also mounted on said frame whereby saidpiston imparts angular displacement to the mirror in one plane, meansfor oscillating said frame about the axis of said piston in a planeperpendicular to the first mentioned plane, a source of light directedonto said mirror, means whereby the reflected rays that trace out thediagram are directed during all positions of the mirror to the observerseye, and means for measuring said diagram by observation, for thepurpose specified.

7. in apparatus for optically producing indicator diagrams, thecombination of a conical block having a bore communicating with thesource of pressure, a piston located in said bore, a frame mounted onsaid block, a mirror pivotally mounted on said frame, a spring mountedtransversely in said frame so as to bear loosely on said piston and bedeflected thereby, means for oscillating said frame about the axis ofsaid piston in one plane, means whereby said spring imparts angulardisplacement to the mirror in a plane perpendicular to the firstmentioned plane, a source of light directed onto said mirror, and meanswhereby the reflected rays that trace out the diagram are directedduring all positions of the mirror to the observers eye, for the purposespecified.

8. In apparatus for optically reducing indicator diagrams, thecombination of a conical block having a bore communicating with thesource of pressure, a piston located in said bore, a frame mounted onsaid block, a mirror pivotally mounted on said frame, a spring mountedtransversely in said frame so as to engage with said piston and bedeflected thereby, means for oscillating said frame about the axis ofsaid piston in one plane, means whereby said spring imparts angulardisplacement to the mirror in a plane perpendicular to the firstmentioned plane, a source of light directed onto said mirror, meanswhereby the reflected rays that trace out the diagram are directedduring all positions of the mirror to the observers eye, and means formeasuring said diagram by observation, for the purpose specified.

9. In apparatus for optically producing indicator diagrams, thecombination of a piston subject to varying pressure, a frame mountedaxially with respect to said piston, a mirror pivotally mounted on saidframe, a spring mounted transversely in said frame so as to engage withsaid piston and be deflected thereby, means for oscillating said frameabout tie axis of said piston in one plane, means whereby said springimparts angular displacement to the mirror in a plane perpeiiidicular tothe first mentioned plane, a source of light directed onto said mirror,and a lens situated in a position to receive the beam of light reflectedfrom the mirror during all positions of the latter and to direct thesame to the observers eye, for the purpose specified.

10. In apparatus for optically producing indicator diagrams, thecombination of a piston subject to varying pressure, a frame mountedaxially with respect to said piston, a mirror pivotally mounted on saidframe, a spring mounted transversely in said frame so as to engage withsaid piston and be deflected thereby, means for oscillating said frameabout the axis of said piston in one plane, means whereby said springimparts angular displacement to the mirror in a plane perpendicular tothe first mentioned plane, a source of light directed onto said mirror,a lens situated in a position to receive the beam of light reflectedfrom the mirror during all positions of the latter and to direct thesame to the observers eye, and means for measuring said diagram byobservation, for the purpose specified.

11. In apparatus for optically producing indicator dia rams, thecombination of a spring-control ed piston subject to varying pressure, aframe mounted axially with respect to said piston, a mirror pivotallymounted on said frame, means also mounted on said frame whereby saidpiston imparts angular displacement to the mirror in one plane, meansfor oscillating said frame about the axis of said piston in a planeperpendicular to the first mentioned plane, a source of light directedonto said mirror, means whereby the reflected rays that trace out thediagram are directed during all positions of the mirror to the observerseye, and a transparent screen engraved with scales and placed in theplane on which the image of the source of light is formed, for thepurpose s ecified.

12. n apparatus for optically producing indicator diagrams, thecombination of a piston subject to varying pressure, a frame mountedaxially with respect to said piston, a mirror pivotally mounted on saidframe, a spring mounted transversely in said frame so as to engage withsaid piston and be deflected thereby, means for oscillating said frameabout the axis of said piston in one plane, means whereby said springimparts angular displacement to the mirror in a plane perpendicular tothe first mentioned plane, a source of light directed onto said mirror,a lens situated in a osition to receive the beam of light reflectec fromthe mirror during all positions of the latter and to direct the same tothe observers eye, and a transparent screen engraved with scales andplaced in the plane on which the image of the source of light is formed,for the purpose specified.

13. In apparatus for optically producing indicator diagrams, thecombination of a conical block having a bore communicating with thesource of pressure, a piston located in said bore, a frame mounted onsaid block, a mirror pivotally mounted on said frame, a spring mountedtransversely in said frame so as to engage with said piston and bedeflected thereby, means for oscillating said frame about the axis ofthe piston in one plane, means whereby said spring imparts angulardisplacement to the mirror in a lane perpendicular to the firstmentioned p ane, a source of light directed onto said mirror, a convexlens situated in a position to receive the beam of light reflected fromthe mirror during all positions ofthe latter and to direct the same tothe observers eye, and a transparent screen engraved with scales andplaced in the lane on which the image of the source of light is formed,for the purpose specified.

1a. In apparatus for optically producing indicator diagrams, thecombination of a spring-controlled piston subject to the varyingpressure in the interior of an internal combustion engine, a framemounted axially with respect to said piston, a mirror pivotally mountedon said frame, means also mounted on said frame whereby said pistonimparts angular displacement to the mirror in one plane, means wherebysaid mirror is rocked about the axis of said piston in a planeperpendicular to the first mentioned plane, a source of light directedonto said mirror, means whereby the reflected rays that trace out thediagram are directed during all positions of the mirror to the observer's eye, and an electric spark gap arranged in series relationshipwith the igniting spark and placed in close proximity to the source oflight, for the purpose specified,

15. In apparatus for optically producing indicator diagrams, thecombination of a piston subject to the varying pressure in the interiorof an internal combustion engine, a frame mounted axially with respectto said piston, a mirror pivotally mounted on said frame, a springmounted transversely in said frame so as to engage with said piston andbe deflected thereby, means for oscillating said frame about the axis ofsaid piston in one plane, means whereby said spring imparts angular dislacement to the mirror in a plane perpendicu ar to the first mentionedplane, a source of light directed onto said mirror, means whereby thereflected rays that trace out the diagram are directed during allpositions of the mirror to the observers eye, means for measuring saiddiagram by observation, and an electric spark gap arranged in seriesrelationship with the igniting spark and placed in close proximity tothe source of light, for the purpose specified.

16. In a paratus for optically producing indicator diagrams, thecombination of a conical block having a bore communicating with thesource of pressure, a piston located in said bore, a frame mounted onsaid block, a mirror pivotally mounted on said frame, a spring mountedtransversely in said frame so as to engage with said piston and bedeflected thereby, means for oscillating said frame about the axis ofthe piston in one plane, means whereby said spring imparts angulardisplacement to the mirror in a plane perpendicular to the firstmentioned plane, a source of light directed onto said mirror, meanswhereby the reflected rays that trace out the diagram are directedduring all positions of the mirror to the observers eye, means formeasuring said diagram by observation, and an electric spark gaparranged in series relationship with the igniting spark and placed inclose proximity to the source of light, for the purpose specified.

In testimony whereof I aflix my signature in presence of two witnesses.

BERTRAM HOPKINSON.

Witnesses T. A. G. COOHERTON, E. FREEMAN.

